A tension-adjusting device of the above-described type is disclosed in DE-GM No. 8120032, in which a rolling surface rotates on a support by means of a ball bearing. The support is essentially cup-shaped and has an eccentric bore offset from the axis of rotation. The tension-adjusting device swivels and is fastened to a machine frame by means of a mounting bolt. To effect swiveling, working surfaces for an adjusting tool are provided.
Another tension-adjusting device is disclosed in DE-GM No. 8133386, in which a tension roller is fastened to a carrier plate. The swivel point lies outside the tension roller, whereby a greater range of adjustment is achieved. The tension-adjusting device is fastened to a machine frame by a mounting bolt which is also arranged outside of the tension roller.
Both of the above-described prior art devices have the disadvantage that after loosening of the mounting bolt, the tension roller is left to be easily and freely movable throughout its range of adjustment. In particular, in the case of a tension roller employed with a stretched driving belt, the tension roller automatically swivels back immediately upon loosening of the mounting bolt as a result of the tension of the driving belt. When in the case of the known tension-adjusting devices, for example, a slight adjustment is necessary, the swiveling movement must be carefully carried out with two hands and two tools, namely an adjusting tool for swiveling the tension roller and a spanner for simultaneously tightening the mounting bolt. The same applies when measuring instruments are used to determine the tension of the driving belt.